Complying with federal guidelines for healthy meals in schools helps children to stay leaner, according to the latest study on school lunches.

While the U.S. Department of Agriculture (USDA) sets federal nutritional standards for school lunches, states vary in how strictly they adhere to the guidelines, and, in recent years, nutrition experts have criticized the National School Lunch Program (NSLP), begun in 1946 to provide free or discounted lunch to students from low-income households, with doing a poor job of meeting nutritional requirements. Some states passed laws to require NSLPs in their jurisdictions to exceed the USDA’s standards, while others failed to even meet the government’s standards completely. And in a study published in JAMA Pediatrics, researchers from the University of Illinois at Chicago show how important school lunches can be in helping children to maintain healthy weights. The scientists found that children attending schools where lunches exceeded the USDA standards were leaner compared to those in schools falling short of USDA nutritional guides.

The researchers studied 4,870 eighth-graders in 40 states during the 2006-2007 school year, and found that those attending schools that did not exceed nutritional standards were twice as likely to be obese as those in schools where these standards were surpassed. In fact, at schools where meals exceeded USDA nutritional guides, the students eating the free or discounted meals were 12.3% less likely to be obese than those who didn’t eat the lunches. Since low-income kids are at a higher risk of developing obesity, the researchers focused on this population of students and found that stricter nutritional standards were particularly effective in controlling weight.

The NSLP currently feeds over 31 million students each day, and in response to concerns that the lunches do not meet all of the government’s nutritional standards, in January 2012 the USDA revised school meal guidelines by requiring more fruits and vegetables, whole grains and stricter calorie limits. While the current study was conducted before these stricter guidelines were implemented, the schools that exceeded the older USDA standards were essentially enforcing these latest changes.

And so far, the data suggests the new guidelines are on the right track. Students on the NSLP in states with more stringent meal standards tended to be lighter than those in states where the nutritional guides were not followed as faithfully, as well as students not on assisted meal plans.

Even more encouraging, say the researchers, children attending schools with the stricter food standards did not seem to compensate for any potential lack of calories, taste or need for sugar and fat by purchasing more snacks or soda at vending machines or fast food restaurants around the schools.

The results strongly highlight how important school meals can be in ensuring the health of children, and in potentially combating the childhood obesity epidemic. In an accompanying editorial, Marion Nestle, a nutrition professor at New York University and advocate for improving food in schools, wrote “Increasing evidence confirms that school-based dietary interventions can help promote healthier eating patterns and body weights, especially among children likely to bear the greatest consequences of obesity.” And the lower weight recorded among students benefiting from meals that mirror current USDA standards should bolster efforts to continue to hold the NSLP to higher nutritional standards.

But Nestle argues, politics, in the form of food industry interests, has profoundly influenced school lunch changes, and when potato and pizza producers argued that the new rules would damage sales, Congress prevented the USDA from setting limits on potato servings and the amount of tomato sauce needed to qualify as a vegetable.

On her blog, Food Politics, Nestle makes the case that, as the latest study suggests, stricter regulation of lunch programs is worth considering:

The food industry cannot make significant changes on its own. Food companies are beholden to stockholders and returns to investors. We can’t count on consumer demand. It’s up against the billions of dollars spent on food marketing, advertising, and lobbying.It’s government’s role to level the playing field. Studies like this one are beginning to show positive results.

If you take junk food and sodas out of schools, kids don’t eat as much of them and are healthier. If you have strict nutrition standards for school food, the food is healthier and so are the kids. This may all seem self-evident, but now we have research to prove it. Government agencies should pay close attention and figure out everything they can do to make the food environment healthier for children and adults.

However, like the previous standards, the stricter nutritional guidelines adopted by the USDA in 2012 triggered its own flood of complaints, most notably from students who said requirements to reduce the total calories in school lunches left them hungry. In response, the USDA allowed schools more flexibility in meat and grain servings, and public health experts, nutritionists and policy makers continue to debate how permanent this leeway should be.

While it’s clear that nutritional standards for school lunches will continue to evolve, the data show that providing children with healthier options may actually be an effective way of both controlling their weight and curbing their desire to eat less healthy foods. And that, say nutrition and child health experts, is good news.